The tempation, of course, is to argue that Clark takes equally from Dean and Kerry, but that conclusion doesn't necessary follow. Without leaners, Dean and Kerry lose a combined 13 points, while Clark garners only five.

The pollsters should've asked the same darn questions, first with Clark, and then without. It would allowed us to compare apples and apples.

In other words, this paragraph is simply wrong:

But in an unusually compressed nomination process that has defied expectations so far, some of the polling results suggested that the landscape may shift again before the primary election. Asked how they would vote if retired General Wesley Clark entered the race, the number of undecided voters jumped from 9 percent to 23 percent (and 5 percent said they would probably vote for Clark, who has said he will announce his intentions in the coming weeks).

The reason the undecideds rise, as I note above, is because the pollsters eliminated "leaners", not because Clark would create a huge shift in the dynamics of the race.

Now, I don't doubt Clark's entry will shift those dynamics, but it will do so because of the intense media attention it will generate. There's no way any poll could measure that effect before the fact.